In message or data transmission, it is desirable to obtain as uncorrupted a transmission of the user signals as possible, to suppress interference which exists at the same time and in the same frequency band in addition to the wanted signal, and thermal noise, as well as possible in the receivers. To be able to selectively take measures against interference, it is required to know as much as possible about the characteristics of the interference. Apart from the intensity of the interference, such characteristics are, for example, its spectrum, its correlation properties and the direction of incidence of the interfering signals at the receiver.
In many cases such as, for example, in permanently installed radio transmission links, potential interfering influences by other permanently installed transmitters, which do not emit any user signals from the point of view of the transmission link under consideration, are known. According to the prior art, such interfering influences can be suppressed by simple measures such as directional transmission and reception, a procedure normally used in microwave radio. In many cases, especially in the multi-subscriber systems of mobile communication, such information on the properties of the interference is not known. Accordingly, countermeasures adapted to the interference cannot be easily taken. Assuming interference-limited multi-subscriber systems in which, therefore, the interference is essentially caused by other users of one's own system, the time correlation of the interfering signals is equal to the time correlation of the wanted signals and is thus known as long as interfering signals which are incident from different directions are uncorrelated. Knowledge of the time correlation of the interfering signals can be utilized in the receiver for improving the transmission quality by decorrelating the interference.
TD-CDMA as disclosed in A. Klein, P. W. Baier: Linear unibiased data estimation in mobile radio systems applying CDMA. IEEE Journal on Selected Areas in Communications, Vol. 11, 1993, p. 1058 to 1066, as an example for third-generation mobile radio systems, uses the hybrid FDMA/TDMA/CDMA (frequency/time/code division multiple access) method. The time correlation of the interfering signals can be taken into consideration in the data detection. An example in which no information about the correlation properties of the interference are utilized is the WCDMA (wideband CDMA) disclosed in F. Adachi, K. Ohno, A. Higashi, T. Dohi, Y. Okumura: Coherent multicode DS-CDMA mobile radio access DS-CDMA mobile radio system, IEICE Transactions on Communications, Vol. E79-B, No. 9, 1996, p. 1316 to 1324 and F. Adachi, M. Sawahashi: Wideband multi-rate DS-CDMA for next generation mobile communications systems. Proc. IEEE Wireless Communications Conference (WCC '97), Boulder, 1997, p. 57 to 62, air interface concept which is also proposed for third-generation mobile radio systems and which is based on the hybrid FDMA/CDMA multiple access method.
The disadvantageous factor in the transmission methods corresponding to the prior art, is that they do not obtain information on the received interference (or only to a very limited extent). Hence, they do not use such information to a desirable degree for improving the transmission quality. For example, no directional information at all is obtained with respect to the interference. If multiple-antenna receivers are used, directional patterns could be generated. For example, when using array antennas, which selectively have less gain for those directions from which strong interfering signals arrive at the receiver, the ratio between useful power and interference power at the receiver end is maximized. However, this would require knowledge of the directions of interference which cannot be obtained in the systems according to the prior art.
The system described above of the time correlations of the interference, for example in the case of TD-CDMA, are not about obtaining information about the interference. Rather, using knowledge about the interference is questionable, especially in mobile communication, since the instantaneous characteristics of the interference can greatly deviate from those assumed due to the permanent changing in time of the spatial constellation of the mobile stations which, as a rule, is not predictable.
The prerequisite of uncorrelated interference signals arriving at the receiver from different directions, which has been addressed above, is also not generally met. If the signal of an interference source propagates toward the receiver along a number of paths with different delay, and/or if the interfering signals coming from one interference source have different directions of incidence at the location of the receiver, the aggregate interference signal produced by superposition of the interference signals at the receiving location have different time correlations than the individual interference signals. Thus, they also have different time correlations from those of the user signal which have been assumed.